1. Field of the Invention
The present invention generally relates to a Film Deposition apparatus and, more particularly, to a Film Deposition apparatus of a type utilizing the metal organic chemical vapor deposition (MOCVD) technique.
2. Description of the Prior Art
The Film Deposition apparatus of the kind referred to above, that is, the MOCVD-based Film Deposition apparatus is largely employed in the production of semiconductor lasers that can be utilized as a light source for an optical communication system and various information processing devices. With this MOCVD-based Film Deposition apparatus, a semiconductor film of a uniform thickness and a uniform composition can be formed substantially uniformly over a relatively large surface substrate by the thermal decomposition of gaseous materials which leads to crystal growth on the substrate.
The prior art MOCVD-based Film Deposition apparatus will now be discussed with particular reference to FIGS. 5 and 6. The prior art Film Deposition apparatus, generally identified by 30, comprises a chamber-defining structure 2c having a reaction chamber defined therein, a gas introducing unit 3 and a preparatory chamber 10. To form a semiconductor film on a semiconductor wafer within the MOCVD-based Film Deposition apparatus 30, a wafer carrier having at least one semiconductor wafer mounted thereon is placed inside the preparatory chamber 10 which is subsequently evacuated to a substantial vacuum. A gate 9 is thereafter opened to load the wafer carrier 5 into the reaction chamber so as to rest on a substrate susceptor 4 within the reaction chamber.
After the placement of the wafer carrier 5 on the substrate susceptor 4 within the reaction chamber, the wafer on the wafer carrier 5 is heated to a predetermined temperature by means of a wafer heater 8 while the temperature thereof is monitored by a thermocouple 6, and gaseous raw material is then introduced into the reaction chamber through the gas introducing unit 3 to effect a thermal decomposition of the raw material within the reaction chamber. By this thermal decomposition, the crystal growth is initiated to eventually form a semiconductor film on one surface of the wafer.
Specifically, when an InP film is desired to be on the semiconductor wafer, the wafer within the reaction chamber of the chamber-defining structure 2c is heated to a temperature generally within the range of 600 to 700.degree. C. and a mixture of phosphine (PH.sub.3) and trimethyindium (TMI) ((CH.sub.3).sub.3 In) as the gaseous raw material is introduced into the reaction chamber.
At this time, by the effect of heat from the heater 8, the wall of the chamber-defining structure 2c that defines the reaction chamber, as measured at an outer wall surface thereof by means of a thermocouple 13, is also heated to a temperature of about 350.degree. C. and, therefore, a wall film 11 resulting from the thermal decomposition of the gaseous raw material tends to be deposited on an inner wall surface of the chamber-defining structure 2c.
After the formation of the desired semiconductor film on the wafer, the wafer heater 8 is deenergized to allow the temperature of the wafer to be lowered, followed by movement of the wafer carrier 5 from the reaction chamber into the preparatory chamber 10, thereby completing a cycle of Film Deposition. Deenergization of the wafer heater 8 is accompanied by lowering of the temperature at the wall of the chamber-defining structure 2c and, at the time the wafer carrier 5 is unloaded from the reaction chamber, the temperature at the wall of the chamber-defining structure generally attains about 40.degree. C.
By repeating the Film Deposition cycle described above a number of times, films can be successively formed on a plurality of semiconductor wafers.
The above discussed prior art Film Deposition apparatus has a problem. Specifically, successive formation of the films on the plural wafers accompanying cyclic energization and deenergization of the wafer heater 8 causes the wall of the chamber-defining structure 2c to experience a thermal hysteresis of from 40.degree. C. to 350.degree. C. and then from 350.degree. C. down to 40.degree. C. When the wall temperature of the chamber-defining structure 2c (the temperature measured at the outer wall surface of the chamber-defining structure 2c) is about 40.degree. C., that is, when the wafer carrier 5 carrying the semiconductor wafer is to be loaded into or unloaded from the reaction chamber, the wall film sticking to the inner wall surface is prone to peel off under the influence of the difference in coefficients of thermal expansion of the wall film 11 and the material forming the chamber-defining structure 2c.
The wall film peeling off from the inner wall surface of the chamber-defining structure 2c is generally in the form of particles which, when falling onto the wafer, will disturb the crystal growth, eventually resulting in a reduction in yield of the film-deposited wafers.